netfilter: disallow bpf hook attachment at same priority

netfilter: disallow bpf hook attachment at same priority

This is just to avoid ordering issues between multiple bpf programs,
this could be removed later in case it turns out to be too cautious.

bpf prog could still be shared with non-bpf hook, otherwise we'd have to
make conntrack hook registration fail just because a bpf program has
same priority.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20230421170300.24115-5-fw@strlen.de
Signed-off-by: Alexei Starovoitov <ast@kernel.org>